Apparatus for combined gas-steam-ammonia cycle



April 8, 1969 A. M. SQUIRES 3,436,911

APPARATUS FOR COMBINED GASSTEAM*AMMONIA CYCLE Y n-I W/ e g ,wa-:U9 QQ U .n P` \2\ INVENTOR. m M500/kf:

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APPARATUS FOR COMBINED GAS-STEAM-AMMONIA CYCLE April 8, 1969 A. M. SQUIRES APPARATUS FOR COMBINED GAS-STEAM-AMMONIA CYCLE Filed Jan. 4, 1967 IL-i141. ZJ.

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U.S. Cl. 60-39.18 15 Claims ABSTRACT F THE DISCLOSURE There is provided an improved Rankine steam-ammonia cycle peculiarly suited to the use of a clean tluid fuel, the combustion of the fuel being conducted at high pressure and providing much more than the usual amount of reheat to the expanding steam, so that the steam exhausts from a high-temperature expansion in a highly superheated condition. Boiler feed water is heated by the cooling of both the exhausted steam and atmospheric-pressure combustion gases. The cooled steam is condensed against boiling ammonia, used as cycle iluid in a power cycle discharging heat to the environment. The expanding steam may be reheated both by indirect heat transfer of heat from combustion gases and by direct additions of combustion gases to the steam, but direct additions should be regulated in amount so that the dewpoint of the cooled steam in its mixture with combustion products is higher than 190 F.

Background of the invention This application is a continuation-in-part of my application Ser. No. 380,005, tiled July 2, 1964, now U.S. Parent 3,307,350 (Mar. 7, 1967).

Summary ofthe invention This invention pertains to the production of power from readily available fuels such as heavy fuel oil, coal, or natural gas.

An object of the invention is to provide a power cycle peculiarly adapted to the use of a clean uid fuel, substantially free of sulfur and dust, so that eilluents from the cycle are substantially free of objectionable atmospheric pollutants.

Another object of the invention is to provide a power cycle of such increased eiciency and unusual economy that it is economically attractive to provide ancillary plant in which dirty fuels, such as residual oil and coal, are converted into a clean fluid fuel for use in the new power cycle.

Another object is to provide an improved two-fluid power cycle in which power is generated by steam at high temperature, the steam transferring its latent heat to a bottoming-fluid such as ammonia, which serves as the cycle fluid in a Rankine cycle discharging heat to the environment.

Another object is to provide an improved two-fluid power cycle of the top heat variety, in which the ternperature of steam is raised by direct addition of the products of combustion of a clean fluid fuel with oxygen or air, steam subsequently being condensed by heat exchange against a bottoming-uid.

Another object is to provide a steam-ammonia power cycle peculiarly adapted to the use of a so-called supercharged boiler for raising and superheating steam.

Another object is to provide a steam-ammonia power cycle peculiarly suited for use of waste heat present in ilue gases emerging from a gas-turbine power plant or from a magneto-hydrodynamic electricity generator.

Another object is to provide a steam-ammonia power cycle well suited for use of nuclear heat to raise steam for the cycle, together with use of fossil fuels to provide superheat.

Something like 8O percent of electricity used in the United States is generated by means of a Rankine steam cycle of the following general characteristics: Liquid water is pumped to a high pressure. Heat is added by indirect heat exchange from hot flue gases at atmospheric pressure to raise and to superheat steam. Steam is expanded to a highly subatmospheric pressure in a turbine which exhausts to a condenser. Often there is a reheat at an intermediate pressure, rarely there are two reheats. Steam is extracted from the turbine at a series of intermediate pressures -often six or morefor use in the regenerative heating of boiler feed water; i.e., heat is supplied to the water primarily in form of the latent heat of condensation of the extracted steam. Regenerative heating often carries the temperature of boiler feed water to the neighborhood of 500 F. before any heat is added to the water from the hot flue gases. Heat is recovered from tlue gases at low temperature in a heat exchange with combustion air.

The power industry has been diligent in developing and adopting innovations which have brought about a steady improvement in the economy of power generation by the conventional Rankine steam cycle just described. However, power engineers today are in general agreement that the technology of this cycle is in a highly mature state, and further improvements are not to be expected unless there is a drastic reduction in cost of alloy steels capable of withstanding temperatures above about 1050 F., or unless new steels are developed which can withstand a temperature above about 1200 F. while exposed to liue gases containing ash constituents deriving from the combustion of residual oils and coals.

A major disadvantage of present-day fossil-fuel power technology is the discharge of atmospheric pollutants from plants burning oils and coals. The public is rapidly becoming less tolerant of these pollutants, and the past year has seen an astonishing increase in orders placed for nuclear power generating equipment, an increase commonly said to result from a lack of an economically attractive technique for producing electricially from oil and coal without the discharge of pollutants.

An old idea is to convert oil or coal into a clean fuel gas and to burn the gas in a supercharged steam boilersee, for example TVF: Teknisk-Vetenskaplig Forskning, vol. 32. (1961), pp. 19-26. As an offset against the oil or coal conversion costs, the combination of the conventional Rankine steam cycle with a gas-turbine cycle which incorporates a supercharged boiler can afford savings both in capital and in operating costs. Far less heat-exchange surface is needed, and there is a sharp reduction in boiler cost. Gorzegno and Zoschak [Mechanical Engineering, vol. 87, No. 6 (June 1965), pp. 27-31] estimate the surface at one-third and the cost at one-half, for a supercharged boiler at 8 atmospheres. There is general agreement that the combination costs a little less to build than ordinary plant, and affords a reduction in fuel consumption on the order of .5 percent `with use of presentday steam temperatures.

Higher steam temperatures become attractive, however, if heat is transferred to the steam from clean flue gases at high pressure-see, for example U.S. Patent 3,002,347 (October 1961). The aforementioned reduction in heattransfer surface makes is economic, while the cleanliness of the gas makes it feasible, to specify the use of expensive alloy steel which permit a steam temperature of l200 F. or highera practical impossibility with presently lavailable steels when dirty flue gas is provided.

A problem arises when the conventional Rankine steam cycle is combined with a gas-turbine cycle incorporating a supercharged boiler. Air to the gas-turbine cycle is heated by the work done to compress the air to a high pressure. Hence, cold atmospheric air is not available for heat exchange with flue gases which exhaust from the gas turbine. If all boiler feed water is heated regeneratively against extraction steam to a temperature in the neighborhood of 500 F., there is no use for heat in the iiue gases at temperat-ures below this level. If iiue gases are exhausted lto the atmosphere at this level, the loss of plant etiiciency is significant. Accordingly, prior proposals for the supercharged-boiler combined cycle have often reduced the degree to which boiler feed water is heated regeneratively, so that boiler feed water may receive heat from ue gases at temperatures below 500 F. The steam cycle is thereby said to be degraded. Considerable effort has been expended on finding optimum arrangements for combining regenerative boiler feed water heating with the recovery of heat from ue gases-see, for example, American Power Conference, vol. 24 (1962), pp. S50-370. On the other hand, some proposals have omitted heat 4recovery from liue gases exhausted from the gas turbine. A low stack temperature is achieved by reducing the temperature of gases entering the gas turbine to a degree such that the gas-turbine cycle produces no net powersee, for example, American Society of Mechanical Engineers Paper No. 66-GT/CMC-63 (1966).

I have therefore been surprised to discover that the Rankine steam cycle can be improved upon, for purpose of its combination with a gas-turbine cycle incorporating a supercharged boiler, by eliminating the regenerative heating of boiler feed water altogether.

It was recognized in the early years of this century that a higher steam cycle efficiency could be obtained if steam reheated a number of times during its expansion, the total amount of heat added to the steam in the several reheats being such that the steam emerges from the expansion in a highly superheated condition. German Patent 188,644 (September 1907) and U.S. Patent 892,818 (July 1908) incorporated proposals along these lines, in which the superheated exhaust steam was to be cooled against boiler feed water prior to the steams being condensed in a condenser. These early proposals were defective in two respects: A turbine to exhaust highly superheated steam at a highly sub-atmospheric pressure would be impracticably large, especially for a power station of the large sizes now being built; and the heat exchanger to recover heat from steam at highly sub-atmospheric pressure would be expensive.

I have found that these defects can be remedied by arranging for highly superheated steam to be exhausted from a high-temperature turbine at a good deal higher pressure than that which was contemplated in the early proposals; thereafter, heat is first recovered from the exhaust steam by heat exchange to boiler feed water, and subsequently, the steam is condensed in a further heat exchange against a bottoming-fluid such as ammonia, which is employed as cycle fluid in a Rankine power cycle discharging heat to the environment.

I have found that this arrangement is peculiarly advantageous in combination with a gas-turbine cycle, since it is generally disadvantageous to withdraw steam from the high-temperature turbine for use in the regenerative heating of boiler feed water. Accordingly, the arrangement provides boiler feed water at a relatively cold temperature, which can take up heat from atmosphericpressure iiue gases at low temperature levels. Notice that this is true, surprisingly, in spite of the fact that cold boiler feed water is also employed to take up heat from steam at similarly low temperatures. The ability to em- 4 ploy the cold boiler feed water to take up heat from both the superheated steam and the atmospheric-pressure tiue gases can be understood when it is remembered that the specific heat of water is appreciably greater than the specific heat of steam.

As noted earlier, when a supercharged boiler is fired with a. clean iiuid fuel, there is an incentive to use a higher steam temperature than is ordinarily practicable or economic. It has long been recognized that the conventional Rankine steam cycle surfers a progressive deterioration in efficiency, by comparison with the ideal theoretical Carnot cycle, as the temperature rises above the critical temperature of water, 705 F., because heat is added with a rapid acceleration of temperature. The foregoing arrangement of steam and ammonia power cycles has the advantage that more than the usual amount of heat is added to the steam cycle Huid at temperature approaching the maximum temperature in the cycle. In its combination with a gas-turbine cycle incorporating a supercharged boiler, the degrading of the steam by virtue of intake of `low-temperature heat from tiue gases to the cold boiler feed water can be more than offset by the upgrading brought about by intake of the unusually large amount of high-temperature heat which causes the steam to leave the high-temperature expansion turbine in a highly superheated condition. It will be appreciated that the intake of heat to boiler feed water from the cooling of the superheated exhaust steam does not degrade the steam cycle, since this heat is regenerative with respect to the sensible heat in this steam, just as conventional regenerative boiler feed water heating is regenerative with respect to the latent heat of extraction steam.

According to my invention, there is provided an improved Rankine steam cycle of the type in which heat is added to the steam at one or more intermediate pressures during its expansion from a high initial pressure to a low terminal pressure, at least a significant part of the heat being supplied from gases produced by the combustion of a fuel with a gas containing oxygen, the improvement comprising: supplying the heat to the steam in an amount such that steam exhausted from the expansion is highly superheated, deriving the gas containing oxygen by steps which include the compression of a gas containing at least a major portion of the oxygen to a pressure of at least about 40 pounds per square inch absolute (p.s.i,a.), cooling the combustion gases by heat exchange to boiler feed water, cooling the exhausted steam by heat exchange to boiler feed water, condensing at least a major part of the cooled steam by a further heat exchange to a bottomingiiuid such as ammonia thereby heating and boiling the bottoming-fiuid, expanding the bottoming-fluid vapor through a power-developing expansion turbine, condensing bottoming-fluid effluent from this turbine, and pumping liquiform bottoming-fluid to the aforementioned further heat exchange from condensing steam.

My invention is advantageously used in conjunction with a supercharged boiler in which iue gases are generated by the combustion of a fuel with air compressed to a pressure of at least about 40 p.s.i.a., preferably higher than about 60 p.s.i.a., the flue gases giving up heat to steam while remaining at high pressure, than being expanded in a turbine, and finally being cooled against the cold boiler feed water which the power cycle of this invention provides.

This arrangement is especially suited to the utilization of a clean iiuid fuel, such as natural gas. If only dirty fuels are available, such as residual fuel oil or coal, a clean fluid fuel may advantageously be produced from such dirty fuels in ancillary plant using known processese.g., a combination of a step such as carbonization, gasication, and cracking, to produce a fluid fuel, and steps which remove dust and sulfur from the fluid fuel. A discussion of such processes will be found in my U.S. Patent 3,276,203 (October 1966). I believe that economic circumstances are commonly such that the sale of elei mental sulfur derived from the aforementioned ancillary plant, together with the savings of fuel brought about by use of the more eicient power cycle of this invention, will more than pay the cost of producing a clean fluid fuel for use with the cycle. Wherever such circumstances exist, clean power can be cheaper than the dirty power generated by the present-day technology.

A significant part of the heat added to the steam at one or more intermediate pressures during its expansion may advantageously be supplied in form of the direct addition of products of combustion of a clean fluid fuel with oxygen, air, or air enriched with oxygen, provided one is careful to avoid a situation in which the dewpoint of-steam and combustion products issuing from the steam turbine is too low. I have found that power can be generated economically in the bottoming-tluid cycle only if the dewpoint exceeds a critical value, a value which depends upon circumstances of fuel cost, charge for use of capital, and fraction of time during which one expects to operate the plant. The critical dewpoint is a higher temperature in circumstances of cheap fuel, dear capital, and low use factor. The critical dewpoint is lower under contrary circumstances. I have systematically examined a large number of economic situations, and I have found the critical dewpoint to be above about 190 F. under all situations likely to be encountered. In an embodiment employing direct additions of heat to steam, the bottoming-fluid may often advantageously be boiled at two temperature levels.

My aforementioned patent application, of which this is a continuation-in-part, sets forth a power system in which steam is reheated during its expansion by direct addition of products of combustion of a clean uid fuel with oxygen. I have now discovered that good results are obtained even with use of air of normal oxygen content in the combustion, if care is exercised to maintain the dewpoint of the exhaust steam-and-combustion-product mixture at a temperature above about 190 F. I have also discovered that significantly better results can sometimes be achieved if the steam is reheated 'both Iby indirect exchange of heat from liue gases and by direct addition of combustion products, the former preceding the latter. The improvement is particularly marked if indirect steam reheating is carried to at least about 1200 F., as can be economically accomplished if ue gases are clean and at high pressure.

Many proposals have been made for power systems employing a cycle uid comprising a mixture of steam and combustion products. Most such proposals are best viewed as systems in which steam is added to the combustion products of a gas-turbine power cycle. A modern example of a system of this type is described in American Power Conference, vol. 27 (1965), pp. 484-500. In systems of this type, no condensation of steam is practiced, the steam being thrown away to the atmosphere along with the combustion products in turbine exhaust.

A relatively few proposals have enco-mpassed a step in which steam is condensed from a steam-and-combustionproduct mixture exhausted from a turbine. Most proposals of this type have not contemplated the use of a reheat step. Sometimes the condensation of steam was practiced simply to provide boiler feed water and to get rid of noncondensible to the atmosphere-see, for example, U.S. Patents 2,357,041 (August 1944) and 2,447,696 (August 1948). U.S. Patent 986,308 (March 1911) proposed that condensation be practiced to maintain a sub-atmospheric pressure in the turbine exhaust. The Walter cycle for submarine propulsion-see I. Kenneth Salisbury, Steam Turbines and Their Cycles, John Wiley, 1950, pp. 91-92- used oxygen in a combustion to which liquid water was added, the resulting mixture of steam and combustion products being discharged directly to a jet condenser. U.S. Patents 3,087,304 (April 1963), 3,101,592 (August 1963), 3,134,228 (May 1964), and 3,229,462 (January 1966) provided variants of this cycle. British Patent 13,199

6 (1903) and U.S. Patent 886,274 (April 1908) provided for the condensation of steam from a steam-and-com- |bustion-product mixture exhausted from a turbine at sul stantially atmospheric pressure, the latent heat of condensation being used to boil water at sub-atmospheric pressure. In the British patent, the resulting sub-atmospheric-pressure steam was to serve as bottoming-fluid in an auxiliary Rankine cycle. In the U.S. patent, the lowpressure steam was to be boosted in pressure and to be added to the combustion products ahead of the turbine.

A few proposals have contemplated direct additions of combustion products to effect the reheating of steam to be subsequently condensed in a condenser. U.S\. Patent 2,832,194 (April 1958) proposed an addition of combustion products of a fuel with air to steam at an intermediate stage in its expansion from a high pressure to a highly sub-atmospheric pressure maintained in a condenser. The amount of heat which can advantageously be added to steam in this manner is limited, since the pressure which can be maintained in a condenser of economic size rises sharply with an increase in the amount of non-.condensible gases relative to steam. Austrian Patent 89,426 (September 1922) proposed a system in which the temperature of steam was raised by direct addition of products of combustion of a fuel with air, additions being made both ahead of an expansion turbine and at two intermediate pressures in the expansion. Steam was raised entirely from heat recovered from turbine exhaust, which was at a pressure of 0.25 atmosphere. It is doubtful that this proposed system would be economic under any circumstances, in view of the high cost of the turbine stages at high temperature and sub-atmospheric pressure, of the exchanger recovering heat from turbine exhaust, and of the condenser which the system requires,

The disadvantages of the aforementioned U.S. and Austrian patents were effectively overcome by the proposals 'of the aforementioned U.S. Patent 3,276,203. By using oxygen of a purity higher than about volume percent, say, in the combustion to generate gases for direct reheating of steam, a great deal of heat could be added in these gases, and a striking improvement in cycle eiliciency could be achieved, without undue loss of efficiency on account of the rise in pressure in a condenser of practicable size. The high-temperature expansion of the cycle fluid was terminated at a pressure in the neighborhood of atmospheric, eliminating the costly high-temperature sub-atmospheric turbine stages of the Austrian proposal, and the cycle fluid was further expanded at low temperature after the fluid had been cooled by heat exchange to boiler feed water.

I have now discovered that the disadvantages of the Austrian proposal can also be effectively overcome even when air is used in the combustion for direct heating and reheating of steam, if one designs carefully to provide a steam dewpoint in the turbine exhaust suicient to make it economic to recover heat from condensing steam to a bottoming-iluid. The dewpoint of the exhaust cycle fluid in both U.S. 2,832,194 and Austrian 89,426 was far below F.

Generally speaking, the steam dewpoint of turbine exhaust can be raised and thereby maintained at a suitably high temperature by increasing the turbine-exit pressure, by increasing the concentration of oxygen in air or by employing substantially pure oxygen-a gas containing more than about 85 volume percent oxygen, say-and by measures which reduce the total quantity of heat added in form of the direct addition of products of combustion, viz, employing a greater degree of indirect reheating, using a lower turbine-inlet temperature, and using fewer reheats.

A low dewpoint is economically undesirable for a number of reasons. If steam is condensed at a lower temperature, the bottoming-uid must necessarily be vaporized at lower temperature and pressure, and the elliciency by which latent heat of steam is converted into power via the bottoming-fluid cycle is impaired. This effect is magnified by the fact that presence of non-condensibles in the steam inhibits the transfer of heat from the steam to the lbottoming-fluid, and a larger temperature difference between steam and bottoming-uid is economically required, even though a great deal more surface may economically be provided. Thus, a drop of only a few degrees in steam dewpoint can sometimes require a drop of many degrees in the temperature at which the bottoming-fluid is vaporized. For these several reasons, the capital cost of the bottoming-fiuid cycle equipment rises sharply as steam dewpoint falls; and at a dewpoint below about 190 F., this equipment is uneconomic under all situations -likely to be met.

In an embodiment using only indirect additions of heat, there is an efficiency advantage in exhausting steam flrom the turbine at as low a pressure as possible, but the plant cost becomes greater when lower pressures are used, for two reasons: the final stages of the turbine must be larger and hence more costly, and the exchanger in, which the exhausted steam is cooled against boiler feed water requires more surface if the relative drop in pressure of the steam Iwhile passing through this exchanger is to remain at a constant value. I -believe that it is not advantageous to specify an exhaust pressure below about one-half the pressure in the prevailing atmosphere, and I believe that the preferred pressure, in the more usual circumstances of fuel costs and charges for use of capital, is a pressure in the neighborhood of atmospheric. However, a reduction in capital cost can be brought about by using a pressure higher than atmospheric, and in some cost circumstances-or when a plant is required for operation at a low load factor and the capital cost should be held as small as possible-an appreciably super-atmospheric pressure may be preferred. I believe that a pressure higher than about 100 p.s.i.a. is not advantageous under most economic circumstances likely to be er1- countered.

In an embodiment employing direct additions of heat, it is best to confine ones attention to designs in which the steam turbine exhausts at atmospheric pressure or above. If only a relatively small amount of heat is added directly, any small increase in eiciency brought about through use of a sub-atmospheric pressure is offset by the inconvenience of having to provide a compressor to exhaust noncondensibles to the atmosphere from the heat exchange by which steam is condensed against bottoming-uid. If a relatively 4large amount of heat is added directly, I believe that the efiiciency advantage lies in using atmospheric pressure or higher. Generally speaking, a higher pressure appears to result in a cheaper plant. If the amo-unt of noncondensibles in the stea-m is relatively large and if an appreciably super-atmospheric pressure is used, an expansion turbine is advantageously provided by means of which power may be recovered by expanding non-condensibles from the aforementioned heat exchange to theatmosphere.

The initial steam pressure is advantageously higher than about 2,000 p.s.i.a. At least two reheats to a temperature of at least 1200 F. are preferred, and are easily and economically attainable if a clean fluid fuel is -burned in a supercharged boiler.

Viewed broadly, the cycle of this invention is advantageously used in conjunction with any power system in which air, or another oxygen-containing gas, is supplied to a combustion which is conducted at a high pressure. A gas-turbine cycle incorporating a supercharged boiler, already discussed, is an example of such a power system. Other examples are: a gas-turbine plant which exhausts gases containing oxygen to a combustion taking place in an atmosphere-pressure boiler, and a magnetohydrodynamic electricity generator in which a fuel is burned with air-or, preferably, in which a clean fluid fuel is burned with oxygen or air enriched with oxygen, in accordance with the teachings of my co-pending applica- 8 tion Ser. No. 582,883, filed Sept. 29, 1966, now U.S. Patent 3,324,654 (lune 13, 1967).

Steam may advantageously be raised for the cycle of this invention from heat supplied by a nuclear reactor. Nuclear reactors are best adapted to supplying the relatively low-temperature heat needed to boil water. If a. nuclear reactor is used to raise steam, and if a fossil fuel is used t0 superheat steam for the cycle of this invention, the efficiency of conversion of the energy in the fossil fuel to electricity is outstandingly high.

I now prefer to use ammonia. as the bottoming-fluid. Many other substances are capable of giving satisfactory results, however, such as sulfur dioxide, trichloromonoliuoromethane, and a number of other halogenated hydrocarbons, for example.

Brie)c description of the drawings My invention including various novel features will be more fully understood by reference to the accompanying drawings and the following description of the operation of the alternatives illustrated:

FIG. 1 is a flow diagram which illustrates an embodiment of my invention in which all additions of heat to steam are indirect.

=FIG. 2 is a ow diagram of an embodiment in which direct as well as indirect additions of heat are used.

FIG. 3 is a flow diagram of an embodiment in which direct additions of heat are made in form of products of combustion of methane and air.

Description of the preferred embodiments Reference may now be had to FIG. l, which presents diagrammatically a preferred embodiment of my invention and provides both an understanding of the working of the apparatus indicated therein and also a numerical example, predicated upon the use of methane as fuel, the methane being available at a pressure higher than about 120 p.s.i.a.

Air enters the plant represented by FIG. 1 via line 1 from the atmosphere, which is at 14.17 p.s.i.a. and F., the air flowing at a rate of 2,776,122 pounds per hour (lbs/hn). The air is compressed in compressor 2 to about 120.4 p.s.i.a. Methane is introduced via line i11 at a rate of 153,731 lbs/hr. The methane and air are burned in combustion-chamber 12, depicted as a rectangle containing the letters CC, to form flue gases which are cooled (against water and steam) in heat-exchangers 13, 14, and 15. The flue gases are expanded in power-developing expansion turbine 6 from about 114.4 p.s.i.a. and 1500 F. to about 15 p.s.i.a. and about 847 F. The gases are then cooled in heat-exchangers 22 and 23 (against water) to about 280 F., and in exchanger 24 (against liquid arnmonia) to about 240 F. The gases are discharged via line 25 and via a stack (not shown in FIG. l) to the atmosphere.

Boiler feed water (BFW) at about 210.2 F., flowing at a rate of 1,801,600 lbs/hr., is pumped from about 14.17 p.s.i.a. to about 700 p.s.i.a. in pump 20, and the water is heated in heat-exchangers 17 and 23 (against steam and ue gases respectively) to a temperature between about 400 and 450 F. The BFW is then pumped to about 3,000 p.s.i.a. in pump 21, and is further heated in heat exchangers 16 and 22 (against steam and ue gases respectively). The heated Water is sent to heat-exchanger 13, where it is boiled and the resulting steam is superheated (against flue gases at about p.s.i.a.). Steam is delivered from heat-exchanger 13 to the inlet of expansion turbine 3 at 2,400 p.s.i.a. and 1200" F., and is expanded in this turbine to about 474 p.s.i.a. Steam from turbine 3 is reheated in heat-exchanger 14 (against flue gases) and is delivered to turbine 4 at about 427 p.s.i.a. and 1200 F. The steam is expanded in turbine 4 to about 84.3 p.s.i.a., and is again reheated in heat-exchanger 15 (against flue gases) to 1200 F. The steam from exchanger 15 enters turbine 5 at about 75.9 p.s.i.a. and is expanded in this turbine to about 15 p.s.i.a., leaving the turbine at about 767 F. I believe that best practice of the invention is provided when a temperature at least in the order of about 600 F. is obtained in the highly superheated steam issuing from the high-temperature expansion, terminating in turbine of FIG. l. The low-pressure steam from turbine 5 is cooled in heat-exchangers 16 and 17 (against BFW) to about 260 F., and in exchanger 18 (against liquid ammonia) to about 2l0.2 F. Steam is condensed at about 210.2 F. in heat-exchanger 19 (against liquid ammonia), and BFW at this temperature is furnished to pump 20.

Liquid ammonia at 100 F. and about 211.9 p.s.i.a. is pumped in pump to about 820 p.s.i.a. The ilow rate is about 4,005,600 lbs./hr. The ammonia is heated in heatexchanger 24 (against ilue gas) and in exchangers 18 and 19 (against steam), leaving exchanger 19 as saturated vapor at 200 F. The gasiform ammonia is sent from eX- changer 19 to power-developing expansion turbine 7, which discharges ammonia at 100 F. The discharge is condensed in condenser 9, by heat-exchange against atmospheric cooling water or air, and liquid ammonia is supplied to pump 10.

Turbines 3, 4, 5, 6, and 7 furnish power to drive air compressor 2 and electricity generator 8. The respective turbines, the compressor, and the generator are depicted in FIG. 1 as being linked via a common shaft. Often there is advantage in operating some of the equipment at one speed and some at another, or in driving the air compressor -by means of its own separate driving turbine. Turbine 5, at the ilow rate used in this example, is advantageously built in the form of two separate turbines acting in parallel, and some designers will probably prefer to build compressor 2 in two machines. It will therefore be appreciated that the arrangement depicted in FIG. 1 is highly schematic.

This example, under typical conditions of atmospheric cooling water availability, at typical generator mechanical efficiency, and at typical requirements for auxiliary power needed within the power station, can send out electricity at a rate of about 438,385 kilowatts. At this rate the fuel requirements per kilowatt-hour of electricity sent out-the heat rate-is 8,368.6 British thermal units per `kilowatthour (B.t.u./ kw. h.), reckoned on the higher heating value basis. A conventional plant using present-day technology (burning methane at atmospheric pressure, supplying steam to a turbine at 2,400 p.s.i.a. and 1000 F., employing one reheat to 1000 F., and exhausting at 100 F.) would provide a heat rate not less than about 9,450 B.t.u./kw. h., and would require a ilow of steam not less than about 60 percent greater than the steam ilow of the foregoing example of the invention. The outstanding advantage of the invention is apparent.

There is some gain in plant efficiency if one designs for a sub-atmospheric pressure at the inlet to pump `20, but I believe the overall economic advantage is usually otherwise. A design pressure below about one-half that prevailing in the atmosphere at the exit of turbine 5 is not recommended.

I believed there is a saving in plant capital cost, at expense of a decline in plant efficiency, if a pressure appreciably higher than atmospheric is specified at the outlet of turbine 5. This turbine is thereby made cheaper, and less heat-transfer surface is needed for exchangers 16 and 17. [These remarks need to be qualified by pointing out that a relatively small increase in the pressure at the outlet of turbine 5 may cause a drastic increase in the cost to house exchangers 16, 17, 18, and 19 without a corresponding reduction in other costs] If desired, methane entering via line 11 may advantageously be heated by heat exchanger against a portion of the ilue gases from turbine 6.

FIG. 2 provides a schematic diagram from which one may understand how to effect direct additions of heat to steam used in the cycle of the invention. Combustion products for direct additions of heat are generated by burning a clean fluid fuel with oxygen or air or air enriched in oxygen in combustion-chambers 31, 32, and 33. If oxygen or air enriched in oxygen is used, the oxygen may advantageously be provided by the process of my aforementioned application Ser. No. 582,883.

Steam at high pressure and temperature is furnished from steam-raising-and-superheating step 46 to combustion-chamber 31. The pressure is preferably greater than 2,000 p.s.i.a and the temperature is preferably about 1200 F. A mixture of steam and combustion products, at 1500 F., say, is discharged from combustion-chamber 31 to turbine 36. Discharge from turbine 36, at a lower pressure, is reheated indirectly in step 34 and by addition of combustion products generated in combustionchamber 32. The reheated cycle fluid (steam and combustion products) is further expanded in turbine 37, is again reheated indirectly in step 35 and by addition of combustion products in combustion-chamber 33, and is finally expanded in turbine 38 to a pressure preferably at least a little above atmospheric (often advantageously a good deal above atmospheric). The cycle fluid is cooled in heat-exchangers 47, 50, and 52 (against BFW), the iluid flowing from exchanger 50 to exchanger 52 via line E. The fluid is further cooled in exchangers 59, 60, 62, 63, and 65 (against liquiform bottoming-fluid). The cycle fluid is preferably cooled in exchanger 59 to a temperature close to the steam dewpoint. Water condenses from cycle fluid in exchangers 60, 62, 63, and 65. Exchanger 60 should preferably have sufficient heat-transfer surface to condense only a small percentage of the steam, and the condensate from this exchanger is advantageously cooled in exchanger 61 (against liquiform bottom-fluid) and discarded to the atmosphere via line G. Condensates from exchangers 56, 62, 63, and 65 are combined and injected into the top of gas-liquid contacting tower 55, where the water is contacted in a series of counter-current gasliquid contacting stages with substantially gas-free steam introduced into the bottom of tower 55. Liquid water from the bottom of tower 55 is pumped in pump 54. Steam passing overhead from tower 55 contains carbon dioxide and other gaseous matter stripped from the water entering tower 55; the steam is condensed in heat-exchanger 5'6 (against liquiform bottoming-iluid), and gaseous constituents are discharged via blower 57 and line 58 to a stack (not shown in FIG. 2). The greater part of the liquid water from pump 54 is delivered via line F to exchanger 50 (for heating against cycle fluid) and to BFW heating step 51. A small portion of the water from pump 54 is let down in pressure across valve 53, and is vaporized by heat exchange against cycle fluid in heatexchanger 52, the steam from this heat exchange being introduced into the bottom of tower 55. BFW from exchanger 50 and step 51 is further pumped in pump 49', is further heated in exchanger 47 (against cycle fluid) and in BFW heating step 48, and is sent to steam-raising-andsuperheating step 46. Non-condensible gases, together with a relatively small amount of uncondensed water vapor, emerge from exchanger 65. If turbine 38 discharges at just a little above atmospheric pressure, and if the combined condensates from exchangers 56, 62, 63, and 65 amount to the quantity of BFW required at pump 54, the gaseous discharge from exchanger 65 may be vented directly to the atmosphere. If more BFW is needed, the gaseous discharge from exchanger 65 may advantageously be further cooled by direct contact with cooling water, as follows: The gas is introduced into the bottom of gas-liquid contacting tower 66, which is supplied with a stream of cold water at the top. A portion of the water flowing from the bottom of tower 66 is pumped in pump 67, cooled in exchanger 68 (against atmospheric cooling water, say) and injecttd into the top of tower 66. The remaining water from the bottom of tower 66 is added to condensate streams arising in exchangers 56, 62, 63, and 65. If turbine 38 discharges at just a little above atmospheric pressure, the gas passing overhead from tower 66 may be vented directly to the atmosphere via line D. If turbine 38 discharges at an appreciably higher pressure, the gas in line D is advantageously supplied to reheat step 45, and the reheated gas is expanded through turbine 40 to atmospheric pressure, discharging via line 42 to the stack. The quantity of heat added in reheat step D is advantageously no greater than is required to prevent steam from condensing within turbine 40, a circumstance which would require use of more expensive steels to withstand corrosion arising from the simultaneous presence of liquid water and carbon dioxide. Alternatively, it may sometimes be preferable to omit reheat step 45, so that the effluent from turbine 40 is at subatmospheric temperature and may provide refrigeration.

A variety of sources of heat may be used for steps 34, 35, 45, 46, 48, and 1 in FIG. 2, and I do not wish my ideas to be limited in respect to the origins of the heat.

Bottoming-uid expanded in turbine 39 is condensed in condenser 43 by heat-exchange against atmospheric cooling water, say, and is pumped in pump 44, being delivered via line C to heat-exchanger 59, where the bottomingfluid is heated against cycle fluid. Liquiform bottomingfluid is heated in heat-exchangers 61 (against condensate from exchanger 60) and 65 (against cycle uid). A portion of the heated liquid is further heated and boiled in exchangers 62 and 60, and the vapor is conducted to turbine 39 via line A. A second portion is advantageously reduced in pressure across valve 64 and boiled at lower pressure and correspondingly lower` temperature in exchangers 56 (against overhead gas from tower 55) and 63 (against cycle fluid), the vapor from these exchangers being conducted to an intermediate stage of turbine 39 via. line B.

Turbines 36, 37, 38, 39, and 40 drive electricity generator 41.

FIG. 3 presents diagrammatically a specic embodiment in which air is used in combustion to provide direct additions of heat, in accordance with the schematic diagram of FIG. 2. Certain equipment items in FIG. 3 function substantially in the same manner as corresponding items of FIG. 2, and will not again be described or discussed in full detail. They are combustion-chambers 31, 32, and 33; turbines 36, 37, 38, and 39; electricity generator 41; condenser 43; pumps 44 and 49; heat-exchangers 47 and 50; the lines A, B, C, D, E, F, and G. Heat-exchange step 97 of FIG. 3 comprises equipment items 52 through 68 of FIG. 2, to and from which the flows of the several fluids used are conducted via the aforementioned lines. Turbine 38 discharges lat 15 p.s.i.a., say, and non-condensible gases in line D are conducted directly to a stack.

Air enters from the atmosphere via line 71 and is compressed in compressor 72 to 120.4 p.s.i.a., say. A portion of the discharge from compressor 72 is committed to combustion-chamber 90. A second portion is heated to 1000 F., say, in heat-exchanger 84 (against highpressure tlue gases), and is committed to combustionchamber 33. [Heat-exchanger 84 is to be seen in two places in FIG. 3, viz: it is shown as a heater in the stream of air to combustion-chamber 33, and as a cooler in the stream of high-pressure flue gases arising in combustion-chamber 90. This has been done in order to simplify the piping circuitry and to make the essential flow patterns easier to follow and understand. Most of the heat-exchangers of FIG. 3 have been dealt with in like manner.) A third portion of air is cooled in heatexchanger 73 (against BFW) to 240 F., say, and is further compressed in compressor 74 to 551.6 p.s.i.a., say. Part of the air from compressor 74 is heated in heat-exchanger 85 (against flue gases) to 1000 F., say, and is committed to combustion-chamber 32. The remaining air is cooled in heat-exchanger 75 (against 12 BFW) to 240 F., say, and is still further compressed in compressor 76 to 2,526.3 p.s.i.a., say. Air from compressor 76 is heated in heat-exchanger 86 (against ue gases) to 1000 F., say, and is committed to combustionchamber 31.

Methane, available at 600 p.s.i.a., enters via line 78. A portion is heated in heat-exchangers 79 (against atmospheric-pressure flue gases) and 80 (against high-pressure ilue gases) to 900 F., say. Part `oi this portion of methane is let down in pressure across valve 87 and committed to combustion-chamber la second part is let down in pressure across valve 89 and committed to combustion-chamber 33; a third part is let down in pressure across valve 88 and committed to combustion chamber 32. A second portion of methane from line 78 is compressed in compressor 81 to 2,526.3 p.s.i.a., say, is heated in heat-exchangers 82 (against atmospheric-pressure flue gases) and 83 (against high-pressure flue gases) to 900 F., say, and is committed t-o combustion-chamber 31.

Flue gases from combustion-chamber 90 are cooled in heat-exchangers 91 (against BFW and steam), 92 and 93 (against steam), 86, 85, and 84 (against air), and 83 and 80 (against methane). Flue gases from exchanger 80 enter expansion turbine 77 at 1500o F. and 114.4 p.s.i.a., say. The flue gases are expanded to 15 p.s.i.a., say, in turbine 77; and the expanded gases are cooled in heat-exchangers 94 and 9S (against BFW) and 79 and 82 (against methane). The cooled flue gases are discharged to a stack via line 96.

BWF is provided from step 97 via line F at 180 F., say, and is heated in heat-exchangers 50 (against cycle Huid), 73 and 75 (against air), and 95 (against line gases). The heated BFW is pumped to 3,000 p.s.i.a., say, and is further heated in heat-exchangers 47 (against cycle uid) and 94 (against flue gases). BFW from these latter exchanges is sent to heat-exchanger 91 (against high-pressure ue gases), from which steam is supplied to combustion-chamber 31 at 2,400 p.s.i.a. and 1200 F., say. Cycle fluid from expansion turbine 36 (at 582.3 p.s.i.a., say) is reheated to 1200" F., say, in heat-exchanger 92 (against high-pressure flue gases) land committed to combustion-chamber 32. Cycle fluid from expansion turbine 37 (at 127.1 p.s.i.a., say) is reheated to 1200 F., say, in heat-exchanger 93 (against highpressure flue gases) vand is committed to combustionchamber 33. [Exchangers 91, 92, 93, 94, and 95 of FIG. 3 constitute heating steps 46, 34, 35, 48, and 51 respectively of the general scheme depicted in FIG. 2.] Cycle fluid from each of combustion-chambers 31, 32, and 33 is at 1500 F., say.

Turbines 36, 37, 38, 39, and 77 furnish power to drive electricity generator 41 and air compressors 72, 74, and 76.

I have calculated an example based upon FIG. 3 for the pressure and temperature conditions set forth above and subject to the following additional assumptions: Ten moles of air were used per mole of methane in each of combustion-chambers 31, 32, 33, and 90. The relative pressure drop across each of combustion-chambers 31, 32, and 33 was 2.5 percent; the relative drop across each of heat-exchangers 92 and 93 was 10 percent; the relative drop across each of heat-exchangers 85 and 86 was 5 percent, and the relative drop across each of heat-exchangers 73 and 75 was 2.5 percent. Cycle Huid entered -step 97 via line E at 14.17 p.s.i.a. and 250 F., and left step 97 via line D (sufficient BFW having been condensed in step 97) at 14.17 p.s.i.a. Exceeds condensate in line G was 0.5 percent of the BFW flow in line F. Flue gas in line 96 was 270 F. The bottoming-uid was ammonia exhausting from turbine 39 at 100 F. The steam dewpoint of cycle fluid in line E was l98.16 F. Saturated ammonia was supplied to turbine 39 at 175 F. in line A and at 159 F. in line B. The example atfordcd a heat rate of 7970.5 b.t.u./kw. h.

In a second example based upon the arrangement depicted in FIG. 3, items 31, 75, 76, 81, S2, 83, and 86 were omitted-so that two direct additions of heat were provided instead of three. A heat rate of 8092.4 b.t.u./kw. h. was obtained. The exhaust from turbine 36 was at 672 p.s.i.a., exhaust from turbine 37 was at 127.1 p.s.i.a., flue gases in line 96 were at 260 F., cycle Huid in line E was at 240 F., BFW in line F was at 188 F., and the steam dewpoint of cycle uid in line E was 202.82 F. Ammonia in lines A and B was at 181 F. and 167.5" F. respectively.

Comparison of the two foregoing examples might lead to the impression that a lower steam dewpoint in line E is always accompanied by a lower heat rate. That this impression is false can be readily demonstrated. For example, a modification of the second example also omitted exchangers 92 and 93, so that the two reheats were entirely direct. The heat rate of this modification was 8,121.1 B.t.u./kw. h., and the dewpoint of cycle fluid in line E was 196.02 F. Comparison of the rst example based on FIG. 3 with the modication of the second example illustrates the advantage of effecting direct additions of heat at the highest possible temperature levels. For this reason, I believe that direct additions of heat are advantageously accompanied by indirect reheating of the cycle iiuid to the maximum extent possible.

If oxygen or air enriched in oxygen is used in one or more of combustion-chambers 31, 32, and 33, the advantage of providing for indirect as well as direct reheating of the cycle uid lies in a reduction in the losses of cycle eiiiciency arising from the step which provides the oxygen, as well as in the raising of the dewpoint in line E. The advantage is obtained, however, only if a relatively few reheat steps are provided. The advantage of using only a few reheats under certain circumstances, rather than a very large number, was not appreciated at the time of the filing of the aforementioned application of which this is a continuation-in-part.

There is sometimes an advantage in omitting heat-exchangers 73 and 75, and cooling air after compressor 72, as well as after comperssor 74, by injecting water directly into the air leaving the respective compressors.

I have calculated a number of examples in a systematic exploration of the effect of the parent cycle iluid dewpoint in line E upon the economics of power recovery in the ammonia bottoming cycle. Over a wide range of economic conditions-fuel cost, charge for use of capital, anticipated plant use factor- I have found, as a result of these studies, that the use of an ammonia bottoming cycle is economically attractive only at a parent cycle dewpoint above about 190 F. At a lower parent cycle dewpoint, the ammonia power-generating equipment is too expensive. Consider, for example, a series of designs in which the pressure in line E is held fixed and in which the content of non-condensibles in line E is systematically varied. For a given ow of steam in line E, I have found that even the introduction of relatively small amounts of non-condensibles produces a drastic reduction in the ammoniacycle power which can be economically produced. The reduction is largely a result of a drastic lowering in the economically desirable ammonia boiling temperatures, but the reduction is also partly a consequence of the fact that, as non-condensibles increase, a progressively larger proportion of steam latent heat in the cycle uid is not economically recoverable to boiling ammonia. I have found there to be a sharp rise in the heat duty to condense BFW needed by the parent cycle but not provided by condensate from the heating and boiling of ammonia. This is in spite of the fact that it is economically attractive to provide a great deal more heat-transfer surface in the ammonia boiler-exchangers 60, 61, 62, 63, and 65 of FIG. 2.-as the non-condensibles increase in amount. The rise in surface is needed to offset the interference of even relatively small amounts of non-condensible gases with the transfer of heat from condensing steam to the boiling ammonia. At relatively small amounts of noncondensibles, increased yammonia-boiler surface is economically justifiable, up to a limit which is set by the maximum allowable capital outlay for an incremental fuelfree kilowatt of power provided by the ammonia bottoming cycle. As the non-condensibles increase further, however, the per-kilowatt cost of the ammonia power cycle equipment rises because of the decline in ammonia boiling temperatures and the corresponding decrease in power produced per unit of ammonia ow. A point is reached at which it no longer is attractive to provide more surface in the ammonia boiler as the level of non-condensibles rises further. Typically the economically desirable surface area, in a plot versus parent cycle fluid dewpoint, passes through a maximum, and is falling sharply as the dewpoint approaches the lower economic limit, about F.

Other clean gaseous or liquid fuels besides methane may be used in the cycle of my invention, and there is particular advantage in using a clean fluid fuel such as may be derived from residual oils or coals by means of gasification, cracking, carbonization, hydrogasification, hydrocracking, and hydrocarbonization processes and by sulfurand dust-removal processes. Waste heat thrown off from these processes may often be advantageously used to heat BFW, to raise steam, and sometimes even to superheat steam, thereby reducing the duties of the exchangers or steps in FIGS. 1, 2, and 3 which are provided for these respective purposes.

Nuclear heat may advantageously be substituted for a low-temperature portion of the heat supplied to water by exchanger 13 of FIG. 1, or step 46 of FIG. 2, or exchanger 91 of FIG. 3.

Exchangers 13, 14, and 15 of FIG. 1, for example, may be replaced by equivalent exchangers receiving heat from flue gases at about atmospheric pressure, the gases being derived from a magnetohydrodynamic electricity generator or from a combustion to which combustion oxygen is supplied in form of the effluent from a gas turbine. The atmospheric-pressure flue gases would pass from the exchanger substituted for exchanger 15 directly to exchanger 22, turbine 6 being omitted.

Liquiform bottoming-fluid may advantageously be heated regeneratively by one or more extractions from turbine 7 of FIG. 1, for example.

Bottoming-uid may be boiled at more than two temperature levels in -an embodiment like FIG. 2, if desired.

I do not wish my invention to be limited to the particular embodiments described. Higher steam pressure and temperatures will be found advantageous; for example, outstandingly good efficiency will be given by a design in which steam is heated indirectly at 3,500 p.s.i.a. to 1200 F. ahead of turbine 3 of FIG. l, is reheated indirectly to 1300 F. before turbine 4, and to l400 F. before turbine S. Comparable steam conditions may be used in an embodiment like FIG. 3, and a turbine-inlet temperature higher than 1500 F. may advantageously be used in such an embodiment; at such higher turbine-line temperatures, it becomes progressively more advantageous to use oxygen or air enriched in oxygen to maintain a satisfactorily high dewpoint in cycle fluid provided to the bottomingluid cycle.

I intend the expression indirect heat transfer from gases, which appears in some of the appended claims, to embrace the transfer of heat from hot combustion gases to steam via a heat-transfer carrier, such as a molten metal, employed for example in steam power plants described in U.S. Patent 2,902,830 (September '9).

Those skilled in the art will recognize other arrangements and other applications of the invention which will differ from my examples only in detail, not in spirit.

I claim:

1. In apparatus for generating power of the type which includes a pump to pressurize boiler feed water, a generator of steam at high pressure, a series of expansion turbine stages for expanding said steam to a terminal pressure and developing power, substantially all of the exhaust from each nonterminal stage constituting the ow entering the next stage of said series, combustion means for burning a fuel with a gas containing oxygen to produce combustion products, and means for adding heat to said steam at one or more pressures intermediate between said high pressure and Said terminal pressure, said means including one or more heat exchangers each for cooling said combustion products and for heating the exhaust from a stage of said series, the improvement comprising: `(a) one or more heat exchangers for cooling the exhaust from the terminal stage of said series and for heating boiler feed water,

(b) one or more heat exchangers for condensing at least a major part of said cooled exhaust and for heatingand vaporizing a bottoming-fluid such as ammoma,

(c) an expansion turbine developing power and expanding the vapor of said bottoming-fluid,

(d) a condenser to receive the exhaust from said expansion turbine,

(e) a pump to pressurize liquiform bottoming-fluid,

(f) a gas compressor suitable to provide a discharge at a pressure of at least about 40 p.s.i.a.,

(g) means for conducting at least a major portion of said gas to the inlet of said gas compressor,

(h) means for utilizing said discharge to support combustion, said means including said combustion means for burning said fuel with said gas containing oxygen, and

(i) one or more heat exchangers for further cooling said combustion products and for heating boiler feed water.

2. Apparatus of claim 1 in which also said combustion products are at a pressure of at least about 40 p.s.i.a. in said one or more heat exchangers each for heating the exhaust from a stage of said series.

3. Apparatus of claim 2 in which also said gas containing oxygen is atmospheric air.

4. Apparatus of claim 1 in which also said terminal pressure is at a level close to the pressure prevailing in the surrounding atmosphere.

S. Apparatus of claim 1 in which also said terminal pressure is at a level greater than about one-half the pressure in the surrounding atmosphere.

6. Apparatus of claim 1 in which also said terminal pressure is at a level less than about 100 p.s.i.a.

7. Apparatus of claim 1 in which also said fuel is a uid fuel substantially free of sulfur and particulate matter.

8. Apparatus of claim 1 in which also said means for adding heat to said steam at one or more pressures intermediate between said high pressure and said terminal pressure include one or more combustion chambers each for burning a clean fluid fuel with a second gas containing oxygen to form additional combustion products and each fitted with a connection for adding said additional combustion products directly to said steam, the dewpoint of steam in said exhaust from said terminal stage at said terminal pressure being higher than about 190 F.

9. Apparatus of claim 8 including also an expansion turbine developing power and for expanding to the atmosphere the non-condensible gases from said one or more heat exchangers for condensing at least a major part of said cooled exhaust.

10. In apparatus for generating power of the type which includes a pump to pressurize boiler feed water, a generator of steam at high pressure, a series of expansion turbine stages for expanding said steam to a terminal pressure and developing power, substantially all of the exhaust from each nonterminal stage constituting the ow entering the next stage of said series, combustion means for burning a fuel with air to produce combustion products, and means for adding heat to said steam at one or more pressures between said high pressure and said terminal pressure, said means including one or more heat exchangers each for cooling said combustion products and for heating the exhaust from a stage of said series, the improvement comprising:

(a) one or more heat exchangers for cooling the exhaust from the terminal stage of said Series and for heating boiler feed water,

(b) one or more heat exchangers for condensing at least a major part of said cooled exhaust and for heating and vaporizing a bottoming-fluid Such :1S ammonia,

(c) an expansion turbine developing power and expanding the vapor of said bottoming-fluid,

(d) a condenser to receive the exhaust from said expansion turbine,

(e) a pump to pressurize liquiform bottoming-iluid,

(f) a gas compressor suitable to provide a discharge at a pressure of at least about 40 p.s.i.a.,

(g) means for conducting air to the inlet of said gas compressor,

(h) means for conducting said discharge to said combustion means,

(i) an expansion turbine for expanding said combustion products and developing power, and

(j) one or more heat exchangers for further cooling said combustion products and for heating boiler feed water.

11. In apparatus for generating power of the type which includes a pump to pressurize boiler feed water, a generator of steam at high pressure, a series of expansion turbine stages for expanding said steam to a terminal pressure and developing power, substantially all of the exhaust from each nonterminal stage constituting the ow entering the next stage of said series, and means for adding heat to said steam at one or more pressures intermediate between said high pressure and said terminal pressure, the improvement comprising:

(a) one or more heat exchangers for cooling the exhaust from the terminal stage of said series and for heating boiler feed water,

(b) one or more heat exchangers for condensing a major part of said cooled exhaust and for heating and vaporizing a bottoming-fluid such as ammonia,

(c) an expansion turbine developing power and expanding the vapor of said bottomingauid,

(d) a condenser to receive the exhaust from said expansion turbine,

(e) a pump to pressurize liquiform bottoming-fluid,

(f) one or more combustion chambers each for burning a clean fluid fuel with a gas containing oxygen to form combustion products and each tted with a connection for adding said combustion products directly to said steam, each said combustion chamber with its connection comprising at least a significant element of said means for adding heat to said steam, the dewpoint of steam in said exhaust from said terminal stage at said terminal pressure being higher than about 190 F, and

(g) compression means for supplying 4said clean fuel and said gas containing oxygen to said one or more combustion chambers.

12. Apparatus of claim 11 including combustion means for burning a fuel with a second gas containing oxygen to form additional combustion products, and in which also said means for adding heat to said steam include one or more heat exchangers each for cooling said additional combustion products and for heating the exhaust from a stage of said series.

13. Apparatus of claim 11 including also an expansion turbine developing power and for expanding to the atmosphere the non-condensible gases from said one or more 1 7 heat exchangers for condensing a major part of said cooled exhaust.

14. Apparatus of claim 11 in which also at least two heat exchangers are provided for condensing said major part of said cooled exhaust, said bottoming-uid being vaporized at more than one temperature level.

15. Apparatus of claim 11 including also a combustion chamber for burning a clean fluid with a second gas containing oxygen to form combustion products at substantially said high pressure and tted with a connection for adding said combustion products directly to said steam at said high pressure before said steam is introduced into the first stage of said series.

References Cited UNITED STATES PATENTS Tellier 60-36 Abendroth 60-36 XR Pacault et al 60-73 Flatt 60939.18 XR Pacault. Sheldon.

U.S. Cl. XR. 

